Aluminum oxide abrasive and method of making the same



May 7, 1935. J, MASIN 2,000,857

ALUMINUM OXIDE ABRASIVE AND METHOD OF MAKING THE SAME Filed Dec. 1. 1930 INVENTOR BVY M I TORNEYS Patented May 7, 1935 UNITED STATES ALUMINUM OXIDE ABRASIVE AND METHOD OF MAKING THE SAME Jacob S. Masin, Anniston, Ala", assignor to Swann' Research, Incorporated, a corporation of Alabama Application December l, 1930, Serial No. 499,238

4 Claims. (01. 51-279) Myinvention relates to aluminum oxide or alumina for abrasives and for other purposes, and to a method of preparing the same, and has for one of its objects the provision of an economical proc- 5 ess whereby aluminum oxide containing a larger percentage of impurities than has heretofore been thought practicable, may be employed in the arts for the same purposes as pure aluminum oxide.

A further object of my invention is the production of an aluminum oxide containing from 1 to 5% of impurities, particularly TiOz, which shall be superior in its physical characteristics when used as an abrasive to pure aluminum oxide.

A still further object of my invention is the provision of a process for producing aluminum oxide containing from 1 to 5% of TiOz wherein the finished grain will have a toughness comparable to or greater than that of pure aluminum ing wheel, shall be free and cool cutting and which shall not smear or glaze the wheel in-service or require its redressing.

Aluminum oxide, asheretofore produced for commercial purposes, may be' divided'into two general classes, namely, that usedfor more or less rough cutting grinding wheels where there are ather large cuts made in the metal to be ground,

and aluminum oxide prepared for precision work where small cuts are made with the grinding wheel in which it is employed.

For the former purpose; a relatively tough and rugged grain material is required which may contain considerable amounts of impurities and this is usually provided by smelting bauxite with coke in an electric furnace, followed by crushing, separating and screening to the desired size. This treatment may also be followed by other treatments to increase the purity of the grain. In my prior Patent, No. 1,771,570,.i ssued October 7, 1930, there is described and claimed such a process for the production of aluminum oxide and in which I subject the separated grain, after furnace treatment, to leaching with a mixture of hydrofluoric impurities to a minimum. After this treatment, the grain still contains from 1 to 5% of TiOz.

While aluminum oxide grain produced in accordance with my prior patent has proved eminently successful in service and superior to impure aluminum oxide grain theretofore on the market, I have found that where it is embodied in a grinding wheel used for fine precision work, there is a tendency for the wheel to break down too rapidly in service, resulting in a lower production per wheel.

' In my study of the action of grinding wheels, particularly as applied to precision work, I have determined that the major desiderata in a pre- 60 cision grinding wheel grain are the following:-

oxide grain and which, when embodied in a grindand sulphuric or hydrochloric acids to reduce the 1. The grain must be sumciently pure or else the impurities must be rendered innocuous in order to prevent glazing and burning of the wheel in service.

2. The grain must be sufliciently tough so 5 that a too rapid disintegration of..,the wheel does not occur inservice.

3. The grain must be suificiently friable so that a constant, uniform, though a not too rapid breaking down of the grain occurs in service during which breaking down process new sharp cutting edges are uniformly generated to replace those dulled by contact withthe steel being cut.

4. The particles of the grain formed by the breaking down process should be sufiiciently small and friable so that the surface being ground is not deeply gouged as the loose particles pass out from under the revolving wheel.

5. The grain must have good bonding properties. Good bonding properties are manifested by chemical inertness of the grain surface.

0f the-above mentioned desiderata, the proper balance between toughness and friability has hitherto been a diflicult property to obtain, es- 25 pecially in grains made from alumina which contained considerable amounts of impurity.

It has heretofore been the practice, where an aluminum oxide abrasive was to be used for fine precision work, to employ a pure alumina which does not smear or glaze over a grinding wheel because of the absence of associated impurities. Pure alumina, for this purpose, is usually produced by treating bauxite with caustic soda and heating it to bring about a reaction in which sodium aluminate is formed. The latter is filtered oiT in solution and then treated with carbon dioxide or is merely diluted, whereby pure amorphous alumina is precipitated. After separating out the amorphous alumina, thus obtained, and drying it, it is then fused or calcined to form pure aluminum oxide crystals. This product is characterized by being white in appearance and translucent. The product thus obtained is of a high degree of purity and when bonded into grinding wheels, has substantially the physical characteristics pointed out above which are so desirable for precision work.

It is obvious that such a process is a very expensive one and there has resulted areal demand in the art for a cheaper product having a suitable combination of toughness. and friability. Although numerous efforts have heretofore been made to obtain such a cheaper product, none, so far as I am aware, have heretofore been successful.

In approaching the solution of the above mentioned problems I have found that when aluminum oxide prepared in accordance with my prior patent is placed in an oxidizing furnace andsubthe wheel, and that such a wheel does not require frequent redressing.

The treatment of the grain with heat after removing exposed, acid-soluble impurities should not be confused with the ordinary roasting step heretofore practiced in the preparation of aluminum oxide made from fused bauxite or from pure amorphous alumina. Roasting, as heretofore practiced, has usually been at temperatures around 800 or 900 0., whereas I have found it necessary to go to considerably higher temperatures to bring about the required resetting of the crystals, the development of a chemically inert surface, and the proper balance between toughness and friability desired. In general, it may be stated that these changes are evidenced in general appearance by the change in color to white and in the change from translucency to opaqueness.

When examined under the miscroscope, it will be noted that radical changes take place in grain treated in accordance with my improved process. In order that these changes may be better understood reference will now behad to the accompanying drawing, in which:--

Fig. 1 is a view showing a well known pure alumina grain magnified 80 diameters.

Fig. 2 is a similar view of grain treated in accordance with my prior Patent No. 1,777,570.

Fig. 3 is a similar view showing grain treated in accordance with this application; and

Figs. 4, 5 and 6 are views of a grain of aluminum oxide treated in accordance with my present invention magnified over 700 diameters.

In Fig. 1 it will be noted that the grains are irregular in shape, presenting more or less irregular edges. This grain being pure alumina, is translucent and white in appearance.

The grain shown in Fig. 2 has substantially the same outline as that shown in Fig. 1. This grain is made in accordance with my prior Patent No. 1,777,570. The shading of this grain represents impurities in the grain which is translucent, and the impurities show under the microscope to be dissolved in the grain, giving it its pinkish appearance.

In Fig. 3 I show the grain made in accordance with my present invention and magnified 80 diameters, as in Figs. 1 and 2. It will be noted first that this grain, when crushed, breaks along far more irregular lines than the grain illustrated in Figs. 1 and 2. This is an important change in that the grain presents to the work far more sharp cutting edges than either the pure alumina illustrated in Fig. 1 or the alumina prepared in accordancewith my prior patent illustrated in Fig. 2. In addition to the more irregular contour, the grain has become opaque rather than translucent as in Fig. 2 and is white in appearance rather than pink.

n s. 5 and 6, I show grains such as illustrated in Fig. 3 magnified to over 700 diameters. When thus magnified I find that the impurities which before treatment at high temperature were dissolved in the crystals, have become segregated and appear in the treated crystals as rods. extending through the crystals in a plurality of parallel planes. Fig. 4 shows the appearanceof these rods when the crystal is observed from one direction, and Fig. 5 shows their appearance when viewed at right anglesto Fig. 4. It will be seen that in this figure the ends of the rods appear, while in Fig. 4 they appear as straight lines extending across the crystal. In some of the crystals which I have examined under the microscope, I find the rods sticking out of the crystal as in Fig. 6. This appearance is apparently brought about by viewing the crystal in a position intermediate that shown in Figs. 4 and 5.

'This segregation and more thorough oxidation of the impurities in the crystal are the apparent cause of the breakage of the crystals along the irregular lines heretofore mentioned and give it its improved cutting qualities.

In carrying out my improved process, I have found that it is highly desirable to follow the steps outlined in my prior patent, hereinbefore mentioned, followed by the treatment at high temperature as herein disclosed. I have made numerous eiforts without success to produce an abrasive grain of comparable quality, for example, by omitting the step of treating the grain with an acid reagent to reduce the impurities to from 1% to 5%.

The following is a specific example of a preferred method of carrying out my process:

A charge of bauxite and coke, to which iron may be added for the purpose of forming a magnetic alloy with the impurities contained therein, is smelted in an electric furnace so that complete fusion takes place. Time is allowed for the carbon to partly reduce the oxide impurities and segregate to .e bottom of the melted zone in the form of a magnetic iron, silicon and titanium alloy. This operation is referred to as furnacing.

After the melt cools and solidifies, it is broken into lumps, and pieces of refuse and alloyed impurities are picked out. The lumps are then crushed to 6 mesh size or smaller and passed over a magnetic separator to remove the magnetic impurities still remaining.

The material may then be treated according to the procedure outlined in my prior Patent No. 1,777,570, issued October 7, 1930, by which the exposed, acid-soluble impurities in the abrasive grain or crystals are reduced to an optimum amount by an acid treatment such as by treating in a bath of sulfuric acid and hydrofluoric acid. As an illustration of the acid treatment, a lot of graded abrasive is placed in a lead vat containing a bath composed of a mixture of, approximately, 6% hydrofluoric and 16% sulfuric acid. and the grain allowed to remain in contact with the acid for twenty-four hours. The grain is then washed with water and removed from the treating apparatus. It is then dried and screened into sizes in a manner similar to that employed for the other types of abrasive. A typical analysis of the product at this stage of the process may be as follows:

A1203 96.80 TiOz 2.33 S102 .44 F6203 .4

and

" treated is substantially white in appearance.

that a rocking type are furnace is especially suited, although a rotating kiln, heated by means of oil or gas may also be employed. An oxidizing atmosphere is maintained by admitting air or oxygen to the furnace, either continually during the heating operation, or after the material has been heated to the high temperature in a neutral or reducing atmosphere. The product after treatment is removed from the furnace and cooled, after which it may be further sized if necessary. The composition of the grain after heating is approximately the same as before heating.

I have found it necessary to exercise considerable care during the heating operation since the heating may easily be carried too far. I have found that if the heating of the grains containing T102 in the amount specified is carried to the citron-yellow stage described by Mellor in Vol.

VII, page 33, line 6 from bottom of the page, that the oxidation and rearrangement of crystals has progressed to the proper extent and heating may be discontinued. When cold, the grain thus If heating is carried beyond this stage, the color of the grains after cooling becomes more and more brownish, due probably to the formation of titanium nitride. Grains which have been too strongly heated are rendered weak physically.

While I have described a particular acid mixture for treating the grains, other acids may be used. I have found that mineral acids in combination with hydrofluoric acid give good results, in particular such acids as nitric, hydrochloric and phosphoric may be used. It is possible also to use other chemical reagents such as wet chlorine gas, hydrofluoric acid or fluorine gas and even solutions of sodium hydroxide or other alkali hydroxides may be used alone or in combination with other reagents. Hence, I do not desire to limit myself to the use of particular reagents, but contemplate the use of all reagents which will reduce the surface impurities and leave a grain having associated with the alumina the impurities of the amounts and character above set out.

A product treated in accordance with the above process has a toughness factor of 4495x200, whereas aluminum oxide grain tested before being subjected to oxidizing treatment at a high temperature, has a toughness factor of 353012.00. This toughness factor was determined in the following manner:

One gram of aluminum oxide abrasive grain is placed in a steel mortar under a rotating disk or pestle 1" in diameter which weighs a total of 878.5 grams. The grain before testing is first carefully screened through a 32 mesh screen and grains remaining on a 36 mesh screen placed in the mortar forming a layer substantially one grain deep. The pestle is now revolved on the grain at a uniform rate for 100 revolutions, the time for this number of revolutions being about 15 seconds. The contents of the mortar is then screened and the percentage of the grain remaining on a 48 mesh screen expressed as the toughness factor. A variation of about 2 units each way is to be expected and is expresed as a tolerance. The results of the test have been given above.

While I have described my invention in but one form, it will be obvious tothose skilled in the art that it is not so limited, but is susceptible of various changes and modifications, without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are imposed by the prior. art or as are specifically set forth in the appended claims.

What I claim is:

1. An alumina grain composed of fused alumina containing from 1.0 to 5.0% T102, together with small amolmts of silica and iron as impurities, the impurities being segregated in distinct masses within the crystals forming the individual grains thereof.

2. An alumina grain comprised of fused alumina and containing from 1.0% to 5.0% 'IiOz in which the T102 is segregated in distinct rod like masses in the individual grains thereof.

3. An alumina grain comprised of fused alumina and containing from 1.0% to 5.0%"110: in which the TiOz is segregated in distinct rod like masses lying in substantially parallel'planes in the individual grains thereof.

4. An alumina grain comprised of fused alumina and containing from 1.0% to 5.0% TiOz in which the T102 is segregated in distinct rod like masses lying in substantially parallel planes in the individual grains thereof, and the grains are white and substantially opaque when viewed under a miscrosoope. 4

' JACOB S. MASIN. 

